recirculation

The adage that every downside has an upside even applies to the regulations on discharges, which are becoming ever stricter. Full recirculation in sweet pepper crops is now achievable in the lab, and apart from providing a much better understanding of the crop, it can also save money. Growers can already largely avoid emissions without investing in expensive cleaning equipment.

The trial department at the Water Innovation and Demonstration Centre (IDC) in Bleiswijk, the Netherlands, may be only 150 m2 in size but it produced a lot of new information in 2015. In the first trial year the researchers compared zero-emission recirculation in a sweet pepper crop grown on stone wool with conventional cultivation and discharging. A consortium of nine suppliers, the Dutch water boards, the province of South Holland and the Dutch government, are not only financing the research but are also supplying new knowledge and techniques. “Stronger together” is certainly true in this case.
After the successful results achieved in the first trial year, a second trial was run last year, this time with stone wool and coco and with zero emissions. Why coco? Eelke Hempenius of Grodan and Erik van Os of Wageningen University & Research explain: “Following on from the stone wool trial, we also wanted to test other, non-inert substrates. Basically, we need to get a complete picture of zero emission growing.”

Good irrigation water

Coco naturally contains high levels of sodium chloride. At the start of the new season, the drain water already contained 3 mmol/l sodium, compared to less than 1 mmol/l in stone wool. Therefore, the researchers filled the coco slabs with CaNO3 (EC 3.5) for the sodium and calcium exchange and the slabs were buffered.
By always using clean, low-sodium irrigation water throughout the crop cycle, they found that they could obtain almost the same yields with coco substrate as they had with stone wool. The difference was ultimately 6% in favour of stone wool. With the buffered coco slabs, a total of 8 kg nitrate per ha was discharged over the whole crop cycle, excluding the residual water and the slabs. This is a very good result, considering that the total for conventional cultivation in 2015 was 153 kg per hectare.
This positive result provides food for thought. Would it be possible, for example, to meet the discharge requirements without having to spend money on expensive systems? Yes it would, the researchers believe. Hempenius: “Good, low-sodium irrigation water is essential. If you have very good quality water, you don’t actually have to worry about what happens on the backside.” You will need plenty of rainwater storage for that clean water: 1,500 m³ per ha is the recommended amount. Reverse osmosis is also a useful addition.

No need to rinse

Traditionally, growers have drained off the first slab water before planting the crop, in the belief that it contains substances that are detrimental to the young plants’ roots. But according to Hempenius, that isn’t necessary. “Our emission tests have proved this. I have noticed that this idea is still very much ingrained in people’s minds.”
Ingrained – that’s an expression that crops up regularly. The researchers often hear growers saying they prefer not to recirculate because it means the plants are constantly getting the same nutrient solution. That’s an argument Van Os dismisses out of hand. With 30% drain, for example, 70% of the nutrient solution is refreshed, so the plants always get plenty of new nutrients.

Concerns about sodium

By far the biggest concern growers have is that sodium will accumulate in the process water. Sweet pepper is known to be able to tolerate up to around 6 mmol/l sodium, but the figure may be even higher, the researchers claim. Salt tolerance is an area that is not yet fully understood and new research is needed on it. “A lot of what we do is still based on instinct and we tend to err on the side of caution,” Hempenius believes.
This 6 mmol/l sodium level increases the EC by 0.6. With an EC of 2.5, that still allows enough leeway to create a well-balanced nutrient solution. Van Os: “So starting to drain at 4 or 5 mmol/l sodium is really not necessary.”
The nutrient solution must be matched to the plant’s needs, however. During the trial the process water was analysed once a week. In the laboratory of partner Groen Agro Control, uptake was also analysed weekly, based on irrigation, drain, temperature, light sum and CO₂. This shed more light on the plant’s needs. This intensive method of monitoring resulted in a much better match between supply and uptake of nutrients.
Van Os: “By doing this, you also gain in terms of growth and possibly even yields, because you have a better picture of what the plant needs.” He also expects things to move in that direction in practice. Growers who currently only take samples every two or three weeks could substantially increase their sampling frequency. This would result in more growth, less discharge and ultimately a better understanding of the crop, eliminating imbalances in the nutrient solution.

Using up the last residues

In the research into crops grown on stone wool, a great deal of attention was paid to the end of the crop. The aim was to reduce residues from 50 to 20 m³ per ha, without discharging and with no nitrate or phosphate in the residual volume. For example, the total amount of irrigation water was gradually reduced based on the radiation sum.
The nutrient solution also had to be adjusted so that the plants would use up the last remaining nitrate and phosphate residues. This was done by replacing one-third of the nitrate with chloride. The pH was lowered to keep phosphate and trace elements available to the plant. Van Os: “This has to be done very precisely, because you can’t let the crop wilt. Tomato fruits stay firm for quite a while but with sweet pepper, the fruits tend to wilt more quickly than the crop.”
This reduction strategy doesn’t apply to coco. With this substrate, you have to drain to maintain a good nutrient balance right up until the end of the crop.

Affordable solution

Proof that recirculation with very low levels of discharge is possible is not the only insight this research has yielded. For example, the researchers have learned more about the composition of the nutrient solution and have made progress in identifying the plants’ actual needs for nutrients right through the growing season. They have also shown that sweet pepper growers can make substantial savings on water and fertilisers, ranging from five to ten percent of their total water consumption, equivalent to between 400 and 800 m³ per year. This can deliver savings of €2-€3 per m².
By far the greatest gain can be achieved by reducing total emissions by reusing the first water released when the slabs are pierced and from the first drain. During cultivation, smart filter technologies, such as a flat-bed filter, and efficient watering, including the use of a fast ring main, help reduce the total amount of process water. Making these adjustments can save growers from having to purchase expensive cleaning equipment.
And if they do ever need to discharge drain water, they can collect it in a silo and perhaps bring in a contractor to purify it. So compliance with the new regulations doesn’t have to be a costly exercise.

Summary

Having grown sweet pepper on stone wool with zero emissions in 2015, researchers trialled coco and stone wool last year. Stone wool produced good results, with coco also heading in the right direction. What’s more, with a smart watering strategy it is possible to reduce emissions to such an extent that there is no need to invest in costly cleaning equipment. Growers still need to be willing to accept that this is a safe growing method.

Text and images: Pieternel van Velden.